1. Technical Field
The invention relates to techniques for modeling three dimensional objects, and particularly to solid freeform fabrication techniques and objects made through use of such techniques.
2. Related Prior Art
Solid Freeform Fabrication ("SFF") or rapid prototyping techniques are typically useful for quickly making complex or simple three dimensional objects. In general, SFF processes enable rapid and accurate fabrication of three dimensional objects which otherwise could be produced only by lengthy molding and machining processes. SFF techniques are, generally speaking, additive processes whereby the object to be formed is fabricating by reducing a model or representation of the object's ultimate configuration into a series of planar cross-sections and subsequently recompiling the cross-sections to reconstruct the object.
Stereolithography is one of several known SFF techniques. In practicing this process using equipment commonly known as stereolithography apparatus ("SLA"), an ultraviolet laser beam selectively scans a reservoir of a of photosensitive liquid along a predetermined path. Upon the laser beam being exposed to the portions of the liquid lying in the beam's path, the exposed portions of the liquid cure or solidify through polymerization. An example of stereolithographic methods and equipment are disclosed in U.S. Pat. No. 5,256,340, which issued to Allison on Oct. 26, 1993 and which is assigned to 3D Systems, Inc.
Another known SFF process includes Cubital's Solider system. In general, this process utilizes a photo-mask which represents the image of the particular layer of the object to be produced. The mask is positioned over a layer of photosensitive liquid. Selective solidification of the layer occurs upon exposure of ultraviolet light through the mask. Unsolidified resin is drained from the partially composed object leaving the desired configuration of surfaces and cavities. The cavities of the object are then filled with a liquid material having a relatively low melting point, such as wax. Upon solidification of the wax, the uppermost layer of the object is made uniform, such as by planing or milling. Then a new layer of the photocurable liquid is positioned on the surface. Another mask is created and the process is repeated. Upon completion of production, the wax is melted and pour from the object to expose the configuration of the object. As discussed below, the object may comprise a plurality of interconnected, internal cavities or may be hollow.
In addition to these specifically described SFF techniques, there are other techniques not disturbed in detail here. Among these techniques would be plasma deposition techniques hereby plasma is deposited along a predetermined path and permitted to solidify to build an object on a layer by layer basis.
Solid Freeform Fabrication technologies depend on the use of computers to generate cross-sectional patterns representing the layers of the object being formed, and generally require the associated use of a computer and computer-aided design and manufacture (CAD/CAM) software. In general, these techniques rely on the provision of a digital representation of the object to be formed. The digital representation of the object is reduced or "sliced" to a series of cross-sectional layers which can be overlaid to form the object as a whole. The SLA or other apparatus for carrying out the fabrication of the object then utilizes the cross-sectional representations of the object for building the layers of the object by, for example, determining the path of the laser beam in an SLA or the configuration of the mask to be used to selectively expose UV light to photosensitive liquids.
In the normal practice of SFF techniques, because objects or `parts` being fabricating are built on a layer-by-layer basis, where each layer represents a thin cross-section of the part to be formed, is it possible to build solid objects. However, fabricating "solid" parts by completely filling the cross-sectional layers of an object is relatively time consuming and consumes large computing capacities. Also, this approach limits the usefulness of the resultant object by producing an object made entirely of cured photosenstive material, rather than other materials that can be injected into the object.
In the alternative, it is also possible to form hollow structures wherein just the periphery of the object is formed. However, fabrication of hollow objects sometimes is not acceptable because of limitations in the resultant structure and the photosensitive materials used by SLA. In particular, hollow structures often suffer from high structural stresses, shrinkage, curl in the materials and other distortions of the object.
Accordingly, it is also known to form the periphery of the object by formation of a substantially intact boundary or skin, and to provide an integrally formed lattice located internally within the skin boundary. In general, stereolithography is the preferred SFF technique to be used because of its ability to rapidly and accurately fabricate objects of complex geometry with internal, interconnected cavities. An example of such a technique or "build style" is the QuickCast.TM. system by 3D Systems, Inc. which can be used to produce three dimensional objects having a skin and a honeycomb-like internal structure extending between the boundaries defined by the skin.
These known lattice techniques or build styles typically incorporate a construction of cross-hatching in place of completely filling the successive cross-sections. The lattice work of known build styles primarily consist of a cross-hatch pattern of solid material lying in a plane and separated by liquid photopolymer. The outer and inner edges of each layer are solidified by scanning the boundaries of the object to be formed, thus forming the skin.
The desired internal and external object geometry depends upon the anticipated usage of the object formed by the SLA and is based upon a computer generated model or representation of the object. For example, it may be desirable to produce an object with a hollow portions, solid portions and portions occupied by a lattice work. These "build styles" each have distinct advantages and disadvantages. For example, certain build styles, such as the QuickCast.TM. build styles can be useful when the resultant object is to be filled with a material to solidify, strengthen or otherwise further process the object. The presence of a lattice in a build style can often afford more ready introduction of strengthening materials into the object can provide dimensional stability, dimensional accuracy and functionality, or provide a more accurate model of a part being prototyped by use of the SLA.